Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 140333
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
933 PM CST Mon Feb 13 2017

.UPDATE...
Issued at 927 PM CST Mon Feb 13 2017

Already updated the forecast earlier to increase POPS and add
slights along and north of I-70. Well, I will need to increase
the POPS further. Both the MZZU radar and KLSX have shown a
significant increase in Z values of 30+ dbz within the west-east
oriented band located north of I-70. A few of the Missouri mesonet
sites have correspondingly indicated measurable light rain. Mid
level frontogenesis is the main driving mechanism and it is
apparently strong enough to overcome the initially dry low level
air. In fact we`ve had one report of some sleet briefly with
evaporative cooling and the initial onset of precipitation. Later
tonight large scale ascent will spread into southern parts of the
area from the west and this should bring additional rain to that
region.

Glass

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 313 PM CST Mon Feb 13 2017

Sfc ridge will continue to move ewd this evening with winds becoming
swly. Have added some PoPs to mainly the srn third of the CWA for
tonight as the upper trof digs into the area.

Any precip is expected to quickly move out of the area leaving Tues
mainly dry. However, depending on the timing of arrival for a weak
cdfnt late tonight into Tues, precip may linger in the far srn
counties of the CWA.

Generally trended twd warmer temps for tonight with cloud cover
expected to help keep temps warmer. With the arrival of a weak cdfnt
and nwly flow, generally trended twd the cooler guidance for Tues.

Tilly

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 313 PM CST Mon Feb 13 2017

The synoptic pattern at the start of the long term period will
feature a split flow regime, with an active northern stream
affecting mainly the northeastern CONUS, and a weaker southern
stream system moving across the Southern Plains into the Southeast.
The pattern will then transition into a weak Rex Block across the
central CONUS by the end of the week, allowing for a fairly
prolonged period of warm and mostly dry conditions.

The period will start out with a weak vort max/upper low sliding
across portions of Arkansas and Louisiana.  Guidance is in general
agreement that the strong trough diving into the Great Lakes region
will be enough to shunt the moisture associated with the southern
stream system off to the south of the area by Tuesday night.

The trough will amplify over the eastern seaboard on Wednesday,
which will allow for cooler air to spill into the Midwest on its
backside.  Wednesday will likely be our coolest day of the period,
but even those temperatures will be near normal for this time of
year (low to mid 40s).

By Thursday, the eastern trough will have pushed off the eastern
seaboard, allowing for large-scale ridging to build into the central
CONUS atop a weak upper low over northern Mexico/southern TX.  This
will set the stage for a prolonged period of south/southwest surface
flow over the region, allowing temperature to warm into the 60s
Thursday-Sunday, perhaps even the low 70s on any day from Friday
through Sunday. We will have to watch as the weak trough over
Mexico/TX ejects out towards the region Saturday into Sunday as it
could bring some enhanced cloudiness, or perhaps even a sprinkle. A
deeper, larger trough will likely affect the region early next week
(outside the official forecast) which could bring some much-needed
rainfall to portions of the region.  Stay tuned!

KD

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 541 PM CST Mon Feb 13 2017

VFR flight conditions are expected through the forecast period,
with generally overcast mid level clouds tonight and high clouds
on Tuesday. The main question is if any precipitation will occur
at the terminals. I think the best chance of any will be at KCOU
later this evening, and if it occurs, it will be light rain or
sprinkes without any visibility restriction. There is a slight
chance that light rain or sprinkles could extended as far east as
metro St. Louis in the 04-07z time frame, however that probability
is too low to mention in the TAFs.

SPECIFICS FOR KSTL:

VFR flight conditions are expected through the forecast period,
with generally overcast mid level clouds tonight and high clouds
on Tuesday. There is a slight chance for a little light rain or
sprinkles in the 04-07z time frame, however that probability is
too low to mention in the TAFs. Should in fact the light rain
materialize, visibility restrictions are not expected.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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